Guns, Germs & Steel

A suitable starting point from which to compare historical developments on the different continents is around 11,000 BC. This date corresponds approximately to the beginnings of village life in a few parts of the world, the first undisputed peopling of the Americas, the end of the Pleistocene Era and last Ice Age, and the start of what geologists term the Recent Era. Plant and animal domestication began in at least one part of the world within a few thousand years of that date.

Our closest living relatives are three surviving species of great ape: the gorilla, the common chimpanzee, and the pygmy chimpanzee (also known as bonobo). Their confinement to Africa, along with abundant fossil evidence, indicates that the earliest stages of human evolution were also played out in Africa.

Fossils indicate that the evolutionary line leading to us had achieved a substantially upright posture by around 4 million years ago, then began to increase in body size and in relative brain size around 2.5 million years ago. Those protohumans are generally known as Australopithecus africanus, Homo habilis, and Homo erectus, which apparently evolved into each other in that sequence.

All of that human history, for the first 5 or 6 million years after our origins about 7 million years ago, remained confined to Africa. The first human ancestor to spread beyond Africa was Homo erectus, as is attested by fossils discovered on the Southeast Asian island of Java. At present, the earliest unquestioned evidence for humans in Europe stems from around half a million years ago, but there are claims of an earlier presence.⊞ 深度解析 (Unpack)

That illustrates an issue that will recur throughout this book. Whenever some scientist claims to have discovered "the earliest X" — whether X is the earliest human fossil in Europe, the earliest evidence of domesticated corn in Mexico, or the earliest anything anywhere — that announcement challenges other scientists to beat the claim by finding something still earlier.

Human history at last took off around 50,000 years ago, at the time of what I have termed our Great Leap Forward.⊞ 深度解析 (Unpack)

The earliest definite signs of that leap come from East African sites with standardized stone tools and the first preserved jewelry (ostrich-shell beads). Similar developments soon appear in the Near East and in southeastern Europe, then (some 40,000 years ago) in southwestern Europe, where abundant artifacts are associated with fully modern skeletons of people termed Cro-Magnons.

Cro-Magnon garbage heaps yield not only stone tools but also tools of bone, whose suitability for shaping (for instance, into fishhooks) had apparently gone unrecognized by previous humans. Instead of only single-piece tools such as hand-held scrapers, multipiece tools made their appearance; recognizable multipiece weapons at Cro-Magnon sites include harpoons, spear-throwers, and eventually bows and arrows, the precursors of rifles and other multipiece modern weapons.⊞ 深度解析 (Unpack)

The settlement of Australia/New Guinea was perhaps associated with still another big first, besides humans' first use of watercraft and first range extension since reaching Eurasia: the first mass extermination of large animal species by humans. Today, we regard Africa as the continent of big mammals. Australia/New Guinea today has no equally large mammals — but Australia/New Guinea formerly had its own suite of diverse big mammals, including giant kangaroos, rhinolike marsupials called diprotodonts reaching the size of a cow, and a marsupial "leopard". All of those Australian/New Guinean giants — megafauna — disappeared after the arrival of humans.

The near-simultaneous disappearance of so many large species raises an obvious question: what caused it? An obvious possible answer is that they were killed off or else eliminated indirectly by the first arriving humans. Recall that Australian/New Guinean animals had evolved for millions of years in the absence of human hunters.

Hence one hypothesis for the demise of Australia's and New Guinea's giants is that they met the same fate around 40,000 years ago. In contrast, most big mammals of Africa and Eurasia survived into modern times, because they had coevolved with protohumans for hundreds of thousands or millions of years.

However, the overkill hypothesis, as it is termed, has not gone unchallenged for Australia/New Guinea. Critics emphasize that, as yet, no one has documented the bones of an extinct Australian/New Guinean giant with compelling evidence of its having been killed by humans. Defenders of the overkill hypothesis reply: you would hardly expect to find kill sites if the extermination was completed very quickly and long ago. The critics respond with a countertheory: perhaps the giants succumbed instead to a change in climate, such as a severe drought on the already chronically dry Australian continent. The debate goes on.

With the settlement of Australia/New Guinea, humans now occupied three of the five habitable continents. That left only two continents, North America and South America — surely the last ones settled, for the obvious reason that reaching the Americas from the Old World required either boats or else it required the occupation of Siberia in order to cross the Bering land bridge.

The oldest unquestioned human remains in the Americas are at sites in Alaska dated around 12,000 BC, followed by a profusion of sites in the United States and Mexico in the centuries just before 11,000 BC — the Clovis sites. These facts suggest the interpretation that Clovis sites document the Americas' first colonization by people, who quickly multiplied, expanded, and filled the two continents.

If the Americas eventually came to hold hunter-gatherers at an average population density of somewhat under one person per square mile, then the whole area of the Americas would eventually have held about 10 million hunter-gatherers; but even if the initial colonists had consisted of only 100 people and their numbers had increased at a rate of only 1.1 percent per year, the colonists' descendants would have reached that population ceiling of 10 million people within a thousand years.⊞ 深度解析 (Unpack)

What significance, if any, do the continents' differing dates of settlement have for subsequent history? Suppose that a time machine could have transported an archaeologist back in time, for a world tour at around 11,000 BC. Given the state of the world then, could the archaeologist have predicted the sequence in which human societies on the various continents would develop guns, germs, and steel, and thus predicted the state of the world today?

Our archaeologist might have considered the possible advantages of a head start. If that counted for anything, then Africa enjoyed an enormous advantage: at least 5 million more years of separate protohuman existence than on any other continent. Furthermore, human genetic diversity is highest in Africa; perhaps more-diverse humans would collectively produce more-diverse inventions.

The issue remains unresolved. With hindsight, of course, we know that Eurasia was the one. But it turns out that the actual reasons behind the more rapid development of Eurasian societies were not at all the straightforward ones that our imaginary archaeologist of 11,000 BC guessed. The remainder of this book consists of a quest to discover those real reasons.